A guide system for laying a pipeline in a body of water normally comprises a laying ramp, which comprises a frame hinged to the stern of a laying vessel to rotate about an axis of rotation, and at least one supporting device fitted to the frame to support the pipeline in a given configuration.
A laying vessel for laying a pipeline on the bed of a body of water normally comprises a floating body, which in turn comprises a pipeline assembly line, and is connected at the stern to the laying ramp, which defines an extension of the assembly line to lay the pipeline gradually onto the bed of the body of water as the laying vessel moves forward.
In the work configuration, the laying ramp serves to support the pipeline along a path extending partly above and partly below water level. Between the laying vessel and the bed, the pipeline assumes an S-shaped configuration, the radius of curvature of the two curves of which depends on the characteristics of the pipeline, the length and configuration of the laying ramp, and the depth of the body of water. Pipeline laying vessels include pontoons, barges, semisubmersibles and single-hull ships, and are selected according to the depth of the bed, prevailing weather conditions, and the type of project. Pontoons and barges are the vessels most commonly used for laying pipelines in shallow water. The laying ramp is connected to the laying vessel by an actuating assembly known, for example, U.S. Pat. No. 4,257,718; PCT Patent Application No. WO 2008/149210; and PCT Patent Application No. WO 2009/098586 filed by the present Applicant. In the above documents, the second portion of the frame is fitted to and projects from arms connected to the laying vessel.
Given their size and structural design, the laying ramps and vessels described in U.S. Pat. No. 4,257,718; PCT Patent Application No. WO 2008/149210; and PCT Patent Application No. WO 2009/098586 are unsuitable for laying pipelines in shallow water, and particularly on beds of varying topography. More specifically, laying pipelines in shallow water calls for the use of vessels with a very small draught.
The weight of the pipeline and laying ramp may also pose stability problems, which are solved by supporting the free end of the laying ramp with variable-buoyancy floats, as described, for example, in U.S. Pat. No. 4,030,311; U.S. Pat. No. 3,670,511; U.S. Pat. No. 3,606,759; U.S. Pat. No. 3,538,712; and U.S. Pat. No. 3,507,126.
The laying ramp described in these documents, however, is unsuitable for use in very shallow water with a bed of varying topography. That is, laying pipelines in water of an average depth of around 4 meters, with a bed varying topographically between 1.5 and 20 meters, would require relatively very large controlled-buoyancy tanks, and, apart from their size and weight, operation of the tanks to adjust their depth would be incompatible with other pipeline laying operations.
Another known drawback lies in the pipeline, in shallow water, forming two opposite curves along a relatively short length of the pipeline. And all the above drawbacks are further aggravated as the specific weight of the pipeline increases.
U.S. Pat. No. 3,280,571 describes a laying ramp comprising sledges fitted to the free end of the ramp and designed to rest on the bed of the body of water.
Though partly solving certain of the above drawbacks, the laying ramp described in U.S. Pat. No. 3,280,571 is only suitable for use on certain types of beds, and cannot be moved in the opposite to laying direction without running the risk of the sledges becoming bogged down and moving the laying ramp out of line with respect to the laying vessel.
One technical solution proposed by the Applicant in PCT Patent Application No. WO 2011/077238 includes supporting the free end of the laying ramp on a powered trolley designed to move along the bed of the body of water. In this case, however, the angle of the laying ramp with respect to the laying vessel is substantially defined by the topography of the bed of the body of water. As such, the angles formed between the laying vessel, the laying ramp and the bed vary alongside variations in the topography of the bed, and may become critical enough to endanger the integrity of the pipeline as depth increases.
A guide system for supporting a pipeline span and alternative or complementary to the laying ramp comprises at least one variable-buoyancy unit designed to fit to and support the pipeline. An example of a float unit for supporting a pipeline as pipeline is laid inside a trench is described in PCT Patent Application No. WO 2010/122395, which describes a post-trenching method (i.e., a method in which an excavator digs a trench under the pipeline lying on the bed of the body of water, and draws along at least one float unit connectable to the pipeline span to prevent it from overbending).
Another example of a float unit is described in U.S. Pat. No. 4,127,006 and relates to laying a pipeline in deep water from a laying vessel. In this document, the float unit moves along the pipeline by varying its buoyancy and its grip on the pipeline.
Neither of the above proposed solutions provides for effective guidance in shallow water of varying topography, in which the float unit has to move rapidly into the best position to support the pipeline span without slowing down the laying work.